I think the UBI should cover the majority of families but there are still going to be irresponsible people out there. You're definitely right that the children should get out of poverty once they are old enough to receive their own UBI but I think there should still be some sort of program in place to protect the children of irresponsible parents throughout their childhood. I'm not sure of the best way to do that but overall their is probably a net benefit for society as poverty will inevitably lead to crime and physical/mental health issues.